Too much timing?
 

So how much is too much on these big engines? Reason I ask is I had mine set at 8 degrees advanced and the motor was good there, but for some reason I am not getting any Vacuum advance. I can tell this by using a timing light and disconnecting and reconnecting the vacuum hose and it makes absolutely no difference at idle. My 292 I6 would sound completely different when I did that. So my engine has been relying on mechanical advance...which seems to be fine as it's very responsive and pulls nicely.
Today I bumped my timing up to 12 degrees advanced and the motor sounds even better...more responsive all around and seems to idle smoother. I haven't driven it yet, but was wondering...you guys think 12 is to much? I know the factory setting is 6 degrees, but I'm getting no vacuum advance at idle. Carb is a 600 cfm edelbrock and works fine. Engine vacuum is 18 what ya call its. lol

Re: Too much timing?
 

TJ, check out my thread named "Stock 305D Dyno Results". (Sorry I don't know how to insert a tread link inside a post.) I'm still running mine the same as noted within the last post. I do a lot of towing too and it runs great; no pinging, etc.

FYI, I've had my truck for six or seven years and I have the same 'lack of vacuum at the distributor at idle' you've noted. I don't believe I have ever brought it up here, so I'm curious to see what you find out. My intake vacuum is 20.

All other engines I have worked on (I6 & 4 bangers), there is a noticeable difference when disconnecting the vacuum advance line.

Re: Too much timing?
 

Greetings TJ! The diaphragm in your vacuum advance is either cracked, or has a hole in it which will not allow it to work. I don't think 12 dgrees advance is excessive, however, I would replace the vaccum advance and reset the timing to 6 to 8 degrees BTDC.

Re: Too much timing?
 

Thanks for the replies guys! 12 degrees ended up being just a tad too much as I noticed cold starts became more involved. lol So I backed the timing down to somewhere's between 8-10. I'll be sure to check the dizzy and see what's going on, FWIW, it is a brand new one from Autozone.

Re: Too much timing?
 

Alright, so I did a complete read up on that thread and that was some great info. Thanks for taking the time to write that up! Anyhow, When I had my timing set at 12 degrees advanced the first thing I noticed was quicker throttle response for sure. Motor even seemed to pull better, BUT when trying to cold start I was more involved and even when the engine was somewhat warm it was a bit harder to start. So I came home and backed the timing down to around 10. Need to drive it again and see what happens. Motor will probably be close to the same as before as 2 degrees more advanced doesn't make much difference. When advancing the timing I didn't get much of an RPM increase or decrease. I Do still need to check my vacuum advance as it made absolutely No difference at idle. My 292 would idle at 10 degrees without vacuum and 25 with vacuum....my 305 V6 idles at 10 with or without so something must be wrong.

Re: Too much timing?
 

The vacuum advance issue has been puzzling me as well.
I checked the 478 with a hand operated pump before I fired it up, and found that it didn't start to operate until about 24 inches. That got me curious so I checked the 305's and found about the same thing. Basically they never operate.
I hope someone can shed some light on this

Re: Too much timing?
 

On the 305 initial timing is set at 7.5 degrees, the vacuum advance has a total of 18 degrees, starts to move at 5-7 inches of vacuum and achieves full advance at 12.25 - 13.5 inches. The centrifugal advance has a total of 30 degrees, 0-2 degrees at 800 rpm's, 11 degrees at 2000 rpm's and 30 at 3400 rpm's thus giving a potential of 55.5 degrees under full vacuum and an rpm of over 3400. On the 351 centrifugal advance is 26 degrees vacuum advance is 18 degrees and 5 degrees initial for a total of 49 degrees at 3500 rpm's. Hope this clears up any confusion, this is from the factory GMC shop manual.

Re: Too much timing?
 

Thanks Clyde! I'm going to check that dizzy on the engine and see if the pump or something is jammed. Cause I'm not getting advance at all. Recently I looked that the pump's arm while reving the motor and there was no movement at all.

Re: Too much timing?
 

TJ, I haven't noticed any change on how it starts. I drove it several years with the timing set per spec and after I advanced the timing it's much more responsive, yet starts the same. I drive mine during all seasons, even if it's 15 degrees or 100 degrees.

I swung by O'Riellys on my way home tonight to get a new vacuum advance, but they couldn't find the part number, so I need to do some research. If I still have the orig one, I wouldn't be surprised if the diaphram is stiff.

When I was a kid rebuilding honda car engines, an old timer taught me to advance the timing a few degrees, because it would make the engine start quicker. I don't know if it's true or not, but I did it anyway. Typically, I stick to the specs, untill I started playing around with this 305.

Re: Too much timing?
 

I was told to retard the timing for easier starts. :lolsmack2:

Have the initial timing around 10 now and it seems to be a little easier starting. Still got nice throttle response. Weather is real bad so I won't be out there. Ya know I remember putting the vacuum advance off of my 292's dizzy and onto the 305....and it kept advancing and retarding the timing all by itself....I could watch the arm move back and forth and back and forth and the motor never found a happy medium. But this brand new dizzy in.....nothing....no movement period...I can rev the engine all the time...nothing.

Re: Too much timing?
 

TJ, wild and crazy stuff!

Re: Too much timing?
 

For sure. lol I'm gonna go out in the morning if the weather is better and do some checking.

Re: Too much timing?
 

Ok folks, so I have an update. Turns out my new distributor's vacuum advance pump has a hole in it or something as I can blow air through it. So I took the pump off of my 292's dizzy and I now have advance. On my edelbrock there's two vacuum ports...one is timed vacuum and the other is full constant vacuum...I tired the 2nd port first which put the idle advance at around 16 and Man....you talk about that motor really becoming smooth and quiet...it always had a cool lope to it, but once the timing advanced that far the motor was super smooth and quiet. Took it for a test run and it was much peppier and pulled a bit better( I need to get my manifold machined for that 4 barrel because the 4 down to 2 ain't doing it no more.) I then tried it on the 1st port and the engine seemed to like that best. The 1st port offers No vacuum advance at idle, but once you rev it the advance comes in....so I bumped the timing up to 10 degrees and ran it....seemed to be happier. I will be uploading a video that I'll post this afternoon guys. See ya.

Re: Too much timing?
 

Here's the video guys, still doing tuning:

https://www.youtube.com/watch?v=vUX6qsNt2dA

Re: Too much timing?
 

Thanks for sharing TJ. Mine idles a little rough too and I don't have any vacuum advance at idle, so this will motivate me to fix it. :)

Re: Too much timing?
 

Be sure to post your findings! :)

Re: Too much timing?
 

Update: I mentioned before that ever since I've had this truck, it doesn't matter if I disconnect the hard vacuum line to my vacuum advance; the timing remains the same and so does the idle.

Thanks to TJ, I decided to investigate instead of ignore it. Anyway, I disconnected the hard line from the back of the carb and hooked up my vacuum pump to the hard line. I tested the vacuum advance several times and obtained consistent readings. At 5 psi, I noticed the rod in the slot starting to move, and it would hold vacuum. At 14 psi, the rod would stop moving, and it would hold vacuum. I could pump it up a little higher, but the rod quit moving after 14 psi.

So I removed the reducer fitting from the back of the carb to inspect and it was clear. I also looked in the hole in the back of the carb with my inspection mirror and there were no obstructions.

Do my test results seem normal? Any suggestions on additional inspections? Maybe I need to dig a little deeper into the distributor? The mechanical advance weights/springs on the top side (under the rotor) are clean and move easily.

Re: Too much timing?
 

Greetings Clarke!

There's no need to dig into the distributor. That's not where your problem is. The fact that your vacuum pump causes the vacuum advance to move, and that it holds vacuum tells you that your vacuum advance is OK and not your problem. If you connect a vacuum guage to the fitting at the back of the carb, dimes to donuts you will not get a vacuum reading when the engine is running. Check the gasket between the carb and the intake manifold to see if the area in that gasket to that fitting is blocked off. Without being there and not being able to see things firsthand, that would be my suggestion at this point, because there is something that is stopping you from having vacuum to that fitting. Hope this helps!

Re: Too much timing?
 

Thanks for the feedback George.

I'll hook my vacuum gauge to the port on the back of the carb to see if there's anything at all. If not, I'll pull the carb off and check the base as you suggested.

I had the carb rebuilt two years ago, so I would have thought it would have been addressed at that time, but no difference.

New base gaskets were installed at that time. I will check the oreintation of the gaskets in comparison to the bottom of the carb. If I have any questions, I'll post them.


Thanks again,

Clarke

Re: Too much timing?
 

Clarke:
That does sound normal. I didn't have a gauge on mine, but mine does the same. Give it a bit of throttle and the advance arms starts to move....give it more and to a point it stops. And since I'm running a 4 barrel that was on Ported Vacuum. I do need to get another video up. lol Been sick and busy so I haven't gotten around to it.

Re: Too much timing?
 

Greetings TJ!

Sorry to hear that you've been a bit under the weather. And I'm sorry to "rain on your parade" so to speak, but you should have full vacuum at idle where your vacuum advance is connected to. That's why it is recommended when setting your ignition timing that the vacuum advance hose be disconnected from the advance and plugged (temporarily) with the butt end of a drill bit, or some other object to prevent the engine from having a rough idle while setting the timing. If you have vacuum, and your vacuum advance is working properly, you should notice a perceptible increase in engine idle RPM when the vaccum advance is reconnected. You definitely should not have to increase your engine RPM to get the vacuum advance to "move." You should also have no less than 15 inches (PSI is a misnomer here) of vacuum at the fitting that your vacuum advance is connected to. And not to make anyone sound dumb here, but the mistake is made by many people refering to a vacuum reading as "PSI." This was one of my Automotive Technology teacher's pet peeves--refering to vacuum as "PSI" instead of the proper terminology of "15 inches of vacuum."

Re: Too much timing?
 

I REALLY need to get another video up. LOL.
My 4 barrel carb has two vacuum advance ports. Ported and Manifold. Ported allows the engine to idle at initial timing and Manifold vacuum allows the engine to idle at full vacuum advance. I believe I Did post a video on this thread showing the differences. I hooked the advance pump to Manifold advance and the idle rpm increased noticeably and the engine was Much smoother, but I did notice some hesitation coming off idle. Now that probably could have been fixed by retarding the timing a bit. Where I have the motor currently is 10 degrees initial with the advance pump hooked to Ported timing. No advance at idle, but full vacuum advance while cruising down the highway. I may try manifold advance again and retard the timing back to factory specs and see how it likes it, but so far it's been happy. :)

Re: Too much timing?
 

Thanks George, for the clarification on the proper terminology. As I was catching up on messages, I realized I was making the same mistake, and know better. :)

Re: Too much timing?
 

No problem Clarke! I think you will find that your engine will run much smoother and operate more efficiently when your vacuum advance is working properly! :thumbsup:

Re: Too much timing?
 

Update:

With the steel vacuum line (for vacuum advance) disconnected from the back of the carb, I connected my vacuum gauge to the back of the carb and here are the results:

0.8 inches @ 500 RPM
2.0 inches @ 800 RPM
5.0 inches @ 900 RPM

When I tested my vacuum advance the other evening, it started to move at 5.0 inches.

My vacuum reading off the back of the intake manifold (at idle) is consistently 19 inches.

Based on what I provided above, sounds like I'm losing vacuum in the carb?

Re: Too much timing?
 

Hello again Clarke!

What you are describing would be the "ported" vacuum connection that TJ has referred to. On some carbs there are multiple vacuum ports, and most notably on GM (Rochester) carbs, there is a vacuum port on the throttle body, and that is the one that the vacuum advance is connected to since at that location you will have full intake manifold vacuum. Since the GMC Big Block V-6's don't use (for the most part) GM Rochester carbs, your best bet is to tap into the intake manifold to get full vacuum to the vacuum advance. Check your carb to see if there is a port in the throttle body that you can hook up to. You mentioned that your intake vacuum is 19 inches at idle which is good, and if you can get that full vacuum to your vacuum advance, your engine will purr in appreciation! :happy:

Re: Too much timing?
 

Great idea George!

I'm still running the Stromberg carb and I didn't find any other ports to tap into, so I will follow your suggestion and tee into my intake port. It will be interesting to see how different it runs at low RPM.

Thanks a lot George... I learned a lot with your assistance.

And, thanks TJ for bringing this topic up.

Re: Too much timing?
 

There should be two ports on your carb Clarke. One ported and one manifold. My original stromberg has two.

Re: Too much timing?
 

TJ, I looked again and only saw one port. I attached two photos... I guess you have a different carb?

Re: Too much timing?
 

Boy that's interesting. Mine does have two, but maybe it wasn't what I thought. My 305E is a 67 series from what we were able to tell and it had the governed points dizzy on it...two vacuum ports on the vacuum pump....one in the front and one in the back. How it was supposed to work I'm not sure. lol I'll get some pics of my carb for ya later today. The two ports on the carb were both hooked to the pump.

Re: Too much timing?
 

Here ya go Clarke. Mine actually has 3! lol!

Re: Too much timing?
 

Yes, that is interesting. Looks like the carb portion is the same, but your lower attaching plate has two additional ports.

Thanks for sharing.

Re: Too much timing?
 

Might also be cause mine was in a larger series truck. Not sure. But the carb should be the same, just a different base. :)

Re: Too much timing?
 

I have a 305 e Stromberg, they have one port, runs to the distributor.

Re: Too much timing?
 

Wish I didn't get rid of that old dizzy. lol Might have been able to help shed some light on this subject.

Re: Too much timing?
 

Was driving the ol truck around today. V6 Really seems to be happiest with 10 initial and ported vacuum advance. Everything else I tried just Doesn't seem to be as nice. So I'm sticking with this setup. By the way....I had the ol thing up to 70 MPH for a Short time. LOL Bout 3300 RPM or something like that....super scary though...ain't doing that again. I like that motor and want it to last for awhile. 2500 seems to be it's cruising limit so I don't push past that....Most of the time I'm at a Peak of 2400 which is around 50. Get a bunch of tailgaters, but I don't care...it's My truck and I'm not gonna push it past it's limit cause some idiot is in a hurry on a weekend. lol Cold starting has been more involved, but then it suddenly comes to life. The advanced timing has also helped make warm up times shorter. Used to have to let it idle for like 5 minutes, but now I let it high idle on choke for about 3 and just go easy on it until I get to the high way. Doesn't stumble or back fire when it's cold anymore. She's been a good runner for sure. Confidence keeps going up.

Re: Too much timing?
 

I left my timing at 16, which is what I set it up a year or so ago. I plugged the vacuum port on the back of the carb and connected a rubber vacuum hose from the intake manifold to the distributor. The truck idles smooter when I hook the full vacuum up to the distributor, as the timing is further advanced.

Once I get above 800-1000 rpm, it seems to be the same as before.

I consistently run mine at 3K rpm and shift around 3,400 rpm when hauling a load. I still might install a gear splitter one of these days. Shifting from 3rd to 4th it drops 900-1000 rpm and when you're hauling a load, it struggles to maintain.

Re: Too much timing?
 

Greetings Clarke!

16 degrees BTDC (Before Top Dead Center) initial ignition timing is a bit too much in my opinion. I would set the timing to factory spec, and run the vacuum advance with full intake manifold vacuum. That will be where you will get your best performance. You will lose power with ignition timing that is too far ahead as well as with timing that is too late. It will interest you to note that ignition timing that is too late (retarded) will contribute to engine overheating problems. That is why the people who designed the engine designated that the initial ignition timing be just so many degrees (factory spec) BTDC.

Re: Too much timing?
 

X2 with everything George said. When I bumped the timing up to 10 degrees and then hooked the advance pump to Manifold timing...the engine idled Super smooth...as seen in the video I posted....But, it had some hesitation coming off idle and under wot it would back fire(Partly a 4 down to 2 adapter problem as well). I then backed the timing down to 6 initial and manifold advance...still seemed a bit rough...harder starting and not as smooth....I then brought the timing up to 10 initial and ported advance....Bingo...found that sweet spot. Engine is harder cold starting, but when it's warm it only takes 1/2 a crank, and the engine pulls very well through all the ranges and when cruising I have full advance. I do have to wonder if the roughness in your engine has to do with valves or a vacuum leak?

Re: Too much timing?
 

First of all, this is not my area of expertise, so I appreciate your feedback. Given the subject of this thread, I'd like to explain how I arrived at 16 degrees (in lieu of the factory spec of 7.5 degrees).

Typically, I follow specs closely, but there was a previous thread where someone was talking about advancing the timing, which I've heard so many times over the years, but never much detail on how much advance.

Many old timers I've spoken to over the years say they never use a timing light; they adjust the distributor (advancing the timing) until the motor starts to run rough (detonation), then back it off a little. (What's a little? I don't know? LOL!) I tried that with my 305 and it started running rough at 20 degrees, so I backed if off 4 degrees.

Correct me if I'm wrong, the engine achieves the greatest amount of power when the combustion process happens when the piston is at it's highest position (least amount of open area), just as the piston is starting on it's "down stroke". This is why when you advance the timing from factory spec, the engine idle increases. As you are reducing the amount of open area during the combustion process, the increased energy pushes the piston down, resulting in more power.

In the previous thread I mentioned above, I noted I completed several dozen timed runs, which consisted of cruising in 4th gear (I think 40 mph), then flooring the gas pedal and holding it until I hit 70 mph. I experimented with 7.5, 10 and 16 degrees. The times between 7.5 and 10 degrees timing were very consistent, but with 16 degrees timing the times were consistently faster. I duplicated the timed runs the following day with all three timing settings in reverse order and the results were very close to the day before.

Again, I appreciate the feedback. I obviously do not want to cause engine damage if the above justification is not correct.